What Is Knee Swelling?

Knee swelling, often referred to as “water on the knee”, happens when fluid builds up around or inside the knee joint. This can occur suddenly or develop gradually over time. The fluid might be blood, synovial fluid (which normally lubricates the joint), or pus, depending on the cause.

Swelling can affect:

• The front of the knee (around the kneecap)
• Behind the knee (called a Baker’s cyst)
• All around the joint, making the knee look puffy or balloon-like

Common Causes of Knee Swelling

Not all swelling is created equal. Some causes are minor and temporary, while others are more serious and require medical treatment.

1. Injury or Trauma
Sprains, ligament tears (like an ACL injury), meniscus tears, or fractures can cause immediate swelling. Blood may fill the joint space—a condition called hemarthrosis—which is usually painful and restricts movement.

2. Osteoarthritis
Wear and tear of cartilage over time can lead to chronic swelling, stiffness, and pain, especially after activity or prolonged sitting.

3. Rheumatoid Arthritis
This autoimmune condition causes the body to attack its own joint lining, leading to inflammation, pain, and swelling in both knees or multiple joints.

4. Bursitis
The bursa is a small fluid-filled sac that cushions your knee. If it becomes inflamed—due to overuse, kneeling, or infection—it can cause noticeable swelling.

5. Infections
A bacterial infection inside the joint (called septic arthritis) is a medical emergency. It usually causes intense pain, warmth, redness, and fever along with swelling.

6. Gout or Pseudogout
These conditions result from crystal deposits in the joint, leading to sudden and severe swelling, redness, and pain, often at night.

When Is Knee Swelling Serious?

Mild swelling due to overuse or a minor strain may go away with rest, ice, and elevation. However, it’s important to seek medical attention if:

• The swelling comes on suddenly or severely
• Your knee is hot, red, or painful to touch
• You have a fever (may indicate infection)
• You can’t put weight on the leg or feel unstable
• Swelling lasts more than a few days without improving
• You experience repeated episodes of knee swelling

In these cases, the swelling may be a sign of a more serious condition that requires treatment—like a torn ligament, infection, or inflammatory disease.

How Is Knee Swelling Diagnosed?

Doctors usually begin with a physical exam and medical history. They may recommend:

• X-rays (to rule out fractures or arthritis)
• MRI scans (to detect ligament, meniscus, or soft tissue injuries)
• Joint aspiration (removing fluid from the knee to test for infection or crystals)
• Blood tests (to check for inflammatory or autoimmune markers)

Treatment Options

Treatment depends on the underlying cause:

• RICE Method: Rest, Ice, Compression, Elevation – for mild injuries
• Anti-inflammatory medications: Like ibuprofen or naproxen
• Drainage: Removal of fluid via syringe for severe swelling
• Physical therapy: To strengthen muscles around the knee
• Surgery: May be needed for torn ligaments, meniscus damage, or advanced arthritis
• Antibiotics: If an infection is present

Preventing Knee Swelling

Here are a few simple tips:

• Warm up before exercising and cool down afterward
• Avoid overuse by alternating high-impact with low-impact activities
• Maintain a healthy weight to reduce joint stress
• Wear proper shoes and knee support if needed
• Address minor knee injuries early to prevent complications

While not every swollen knee is a medical emergency, it should never be ignored. Swelling is your body’s way of signaling that something isn't quite right. If it’s persistent, painful, or accompanied by other symptoms, it’s best to consult a healthcare professional.

Taking care of your knees now can help you stay active, mobile, and pain-free in the long run.